Insights into photoaging behaviors and mechanisms of biodegradable and conventional microplastics in soil

Understanding the photoaging dynamics of biodegradable and conventional microplastics (MPs) is crucial due to their widespread environmental risks. However, studies on the photoaging behaviors of different MPs in soil ecosystems are limited. This study focused on two representative MPs, conventional polyethylene (PE) and biodegradable poly(butylene adipate-co-terephthalate) (PBAT), examining their photoaging processes in soil. The photoaging performance of these MPs was characterized using indicators like carbonyl index (CI), oxygen-carbon ratio (O/C), weight loss (WL), and water contact angle (CA). The entropy weight method (EWM) was employed to calculate a comprehensive aging index (CAI), quantitatively measuring overall photoaging. The results revealed that PBAT underwent significantly greater photoaging than PE, with the CAI of aged PBAT (0.88) being over 15 times higher than that of aged PE (0.06). Environmental persistent free radicals (EPFRs) were identified as key factors in MPs' photoaging. LC-MS/MS analysis revealed oxygen-containing byproducts and plastic additives, suggesting photodegradation pathways involving chain scission and oxidation. Density functional theory (DFT) highlighted differences in energy gaps and susceptibility to free radical attacks between PE and PBAT. This study not only compares photoaging behaviors but also introduces a novel method for evaluating MPs' aging, providing a basis for assessing ecological risks in soil.